linux-kernel - Re: [PATCH v2 04/12] x86/xen: drop USERGS

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Message-ID: <20201202170823.GF2951@zn.tnic>
Date:   Wed, 2 Dec 2020 18:08:23 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jürgen Groß <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, peterz@...radead.org,
        luto@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call

On Wed, Dec 02, 2020 at 03:48:21PM +0100, Jürgen Groß wrote:
> I wanted to avoid the additional NOPs for the bare metal case.

Yeah, in that case it gets optimized to a single NOP:

[    0.176692] SMP alternatives: ffffffff81a00068: [0:5) optimized NOPs: 0f 1f 44 00 00

which is nopl 0x0(%rax,%rax,1) and I don't think that's noticeable on
modern CPUs where a NOP is basically a rIP increment only and that goes
down the pipe almost for free. :-)

> If you don't mind them I can do as you are suggesting.

Yes pls, I think asm readability is more important than a 5-byte NOP.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette